Document sample usage vanilla, turbolinks, and turbograft

Adding some usage notes in different operating scenarios

cc @pushrax do these look fairly accurate?  I trimmed down the Turbograft usage notes (since we have some re-application of old context stuff, that isn't necessarily directly applicable)

Here's a link to the nicely formatted README that this PR introduces:
https://github.com/Shopify/twine/blob/add-usage-notes/README.md
Is it useful to separate out this variable in the example?